The present invention, in some embodiments thereof, relates to color coating of optical fibers with UV curable inks, and more particularly but not exclusively to an approach to the problem of color coding that allows for distinguishing between more than twelve individual fibers in a bundle.
UV-curable, special coding ink is used for coloring the surface of optical fibers. Optical fibers used in the optical cable require connecting correctly, which means that the same fiber must be identified at both ends. Different colored optical fibers are useful to distinguish between the various optical fibers in the cable. The need for optical fiber identification increases dramatically when optical fiber counts per cable increase from tens to hundreds or even thousands.
For basic coloring, UV-curable ink can provide a quick cure rate, excellent gel resistance, and good adhesion to the surface of the optical fibers. This kind of ink usually consists of a UV-curable prepolymer, pigment or dye, photoinitiator, and other additives.
As long as the number of fibers in a single tube does not exceed 12, a standard color set is available and a single color per fiber per tube is easy to achieve. The present art involves the use of UV-curable inks for coloring optical fibers with a single color for the purpose of visual identification. This is done by adding a thin layer (up to 10 μm) of the UV-curable ink directly on the primary coating of the fiber (o.d. ˜245 μm) at processing speeds of 1000-2000 meters/min, or more recently 1500-3000 meters/minute. This is implementable for use in cable units, specifically loose tubes, containing up to 12 fibers requiring different colors for identification. The reason is that there are 12 internationally accepted colors in the standard which is used in most international and regional standards for coloring.
In fact, the relevant commercial ink suppliers in the field have 16 colors, and optical fiber manufacturers are able to put 16 distinct colors in a single tube. However this solution is non-standard.
To enable identification of more than 12 fibers—or in the non-standard case 16—in a cable unit, the present art either involves placing a thin colored filament around each 12-fiber grouping in the cable element, or involves using a solvent based-ink, on top of or below the UV-curable base color, to impart an additional marking e.g. a ring, stripe or hash, of contrasting color. In order to meet the standards required of the colored fiber, the coloring process must not adversely affect the optical performance, attenuation, strippability of the primary coating of the fiber, splice-ability of the fiber or the visual identification, and both the base color and additional marking must be clear, indelible and must not smudge or smear.
The present art can only achieve the above requirements by either adding additional process stages and or/materials through a second coating run, or by adding the filament as a separate stage, or by drastically reducing production speeds to 150-300 meters/min in one process.
Relevant prior art includes U.S. Pat. No. 6,470,128, to Khudyakov, et al. dated Oct. 22, 2002, which discloses a UV-curable coating composition for optical fiber for a fast cure and with improved adhesion to a glass coated optical fiber. Also of relevance is EP1497237 (B1)—optical fiber with reduced attenuation loss, and EP0501339 (B1)—Method of manufacturing color-coated optical fibers.
A relatively modest coloring operation may color over 1 million kilometers of fiber per year, so that the commercial implications of such additional stages are very large.